The abstract seems pretty clear to me. In essence low cholesterol is associated with even greater risk than high cholesterol. The optimum is somewhere around 200 mg/dL. The study was performed over a large group of Koreans over more than ten years.

The trick to read scientific papers is that unless you're in that specific field, skimming doesn't work. You have to read sentence by sentence and sometimes stop and look up terms you don't understand for the whole to make sense.